This invention relates to the melting furnace of a glassmaking operation, and in particular to a method and apparatus in which guidance means are employed to impart directional stability to the unmelted mass of glass batch material that floats on the surface of the molten glass in the furnace.
The conventional continuous glass melting furnace is provided with an inlet and an outlet at opposite ends, raw, pulverulent batch material being introduced through the inlet, and molten glass being drawn off at the outlet. Heat for melting is typically provided by flames directed above and across the glass. The batch material is deposited onto the surface of the pool of molten glass contained by the furnace and, as it is carried downstream by the general progression of molten glass toward the outlet, forms a layer on the surface known as the "blanket." The batch blanket may extend along the length of the furnace a substantial distance before it begins to break up into discrete agglomerations of batch known as "floaters." In this type of operation, the downstream end of the batch blanket often tends to drift laterally into contact with one of the sidewalls of the furnace, which leads to a number of severely detrimental side effects.
One ensuing problem arises from the fact that the unmelted or partially melted batch material is highly corrosive to the refractory materials from which the furnace walls are made, so that contact between the batch blanket and a sidewall promotes erosion of the sidewall. This erosion is detrimental not only because furnace life is shortened, but also because it causes greater numbers of particles from the walls to enter the molten glass, which, because they are of a different composition and difficult to melt, appear in the final product as inhomogeneities or defects known as "stones."
Another detrimental side effect of the batch blanket drifting against a sidewall is that thermal conditions in the furnace are made unsymmetrical, which in turn leads to the formation of "hot spots" in the molten glass and sets up uneven circulation patterns. The heat input of this type of furnace is usually concentrated at the longitudinal centerline of the furnace where the unmelted batch is normally centered. Thus when the batch blanket shifts to the side, a region of uncovered molten glass at the center can become exposed to extremely high temperatures and become over-heated, forming a hot spot. Because of the high rate of heating, the hot spot exhibits violent, thermally induced convection currents in that region of the glass which can cause contaminants to be stirred up from the bottom and sides of the pool of molten glass, can increase erosion of the sidewall opposite that against which the batch blanket has drifted, and can cause even more batch material to build up against one sidewall. Furthermore, since a substantial portion of the unmelted batch is in the cooler sidewall region when the blanket has drifted and thermal energy is being wasted in overheating some of the molten glass, the batch materials melt more slowly and less thoroughly and the accumulation of unmelted batch along the sidewall against which it has drifted may grow longer and longer, eventually reaching a point where proper melting has not been achieved in glass arriving at the outlet. The net result of batch blanket drift is a serious deterioration in the quality of glass produced and/or a severe reduction in throughput.
Various mechanical pushing or scraping means have been employed in the past for keeping the batch blanket centered in the melting furnace, but these have been found to be not fully satisfactory because their operation is inefficient and their intermittent action does not assure a uniform quality of glass. An example of a pushing device may be seen in U.S. Pat. No. 3,294,506. A more recent proposal is shown in U.S. Pat. No. 3,495,966, wherein cooling means are deployed in the center of the batch blanket to produce currents in the glass that tends to maintain the batch centered in the furnace. That approach, however, requires the use of specially modified batch feeding means and increases operating costs in that it reduces furnace efficiency by removing a large amount of thermal energy from the furnace.
U.S. Pat. No. 2,780,891 discloses the use of submerged obstacles in glass melting furnaces for impeding the movement of floating masses of batch material so as to shorten the melting zone. However, the obstacles are designed to stop both lateral and longitudinal movement of a lump of batch, and are not adaptable to guiding a continuous batch blanket away from the sidewalls as it progresses along the length of a furnace.
A pair of short baffles that engage the sides of a batch blanket are shown in U.S. Pat. No. 3,204,787. These baffles are mounted above the glass line at the ends of water-cooled metal pipes which extend through the inlet opening. The strength limitations of such an arrangement permit the use of only short baffles and restricts their location to the region of the inlet opening. Such baffles serve primarily to prevent batch from accumulating in the corners, and although they may effect some minor lateral restraint of the upstream portion of the blanket, the more unstable downstream portion is left completely free to drift against the sidewalls.
It is also well known to insert electrodes through the walls of glass melting furnaces into the molten glass for the purpose of generating supplemental heat. The modified use of such electrodes as batch blanket guide means forms part of the disclosure of the related U.S. Pat. Application Ser. No. 528,373 of Ronald L. Schwenninger filed on even date herewith, assigned to the assignee of the present application, PPG Industries, Inc., and entitled "Method and Apparatus for Making Molten Glass." Employing electrodes as guide means, however, entails drilling holes in the furnace wall below the glass line so that exterior electrical connections can be made to the electrodes. But in furnaces where supplemental heating is not desired, such an arrangement presents an unnecessary path for wasteful heat loss from the furnace and a potential site for erosion and leakage.
Thus there is a need for means to impart more effective lateral guidance to a continuous batch blanket that does not reduce furnace efficiency or entail the other drawbacks of prior art arrangements.